I think if they self-select to group with their own kind they radicalize each other. But there is not much we can do about that because no matter how many forums get shut down another one will pop up. The community itself doesn't go away just because a platform goes away, that just aggravates them.
The best opportunity we have to help normalize them is while they are on public platforms like twitter, youtube and facebook. Unfortunately those platforms have chosen to ban accounts rather than just censor and throttle content (for reasons that make perfect commercial sense). And good-faith statements of bad ideas are not being engaged with because they get censored as well (obviously bad faith trolling and memes don't invite engagement).
IMHO the best response is to engage where engagement is invited (everyone is edified by such discussions), censor where it isn't invited and where it is otherwise necessary to censor, throttle the ability of repeat offenders to post... but never perma-ban accounts.
In summary I guess my point is this: We can't stop them from choosing to go into a dark corner, but we can stop pushing them there.